In a known piezoelectrically actuated fuel injector, a piezoelectric actuator arrangement is operable to control the position occupied by a control piston, the piston being moveable to control the fuel pressure within a control chamber defined, in part, by a surface associated with the valve needle of the injector to control movement of the injector. The piezoelectric actuator typically includes a stack of piezoelectric elements, the axial length of which is controlled by applying a voltage across the stack through an electrical connector. It is known to arrange the piezoelectric stack within an accumulator volume which is arranged to receive high pressure fuel, in use, so as to apply a hydrostatic load to the stack.
It is important to ensure that the piezoelectric stack and the associated electrical connections are sealed from high pressure fuel within the accumulator volume. For this purpose, it is known to use a plastic over-moulding technique to encapsulate the piezoelectric actuator arrangement within a plastic casing. However, it is also important to ensure that any sealing arrangement which is provided does not significantly increase the size of the actuator arrangement as the accommodation space available for the actuator within the injector is limited. Known over-moulding techniques result in the encapsulated actuator arrangement having a relatively large size.
It is also known to hermetically seal the piezoelectric actuator arrangement within a metal walled container. However this process involves the use of a laser-welding technique to seal a closure plate on the container once the actuator arrangement has been inserted. The method of assembly is therefore a relatively time consuming and costly process. Additionally, when assembled, the actuator arrangement is a relatively large unit, whereas it is an advantage for the unit to be compact.
It is an object of the present invention to provide a method of assembling a piezoelectric actuator arrangement which alleviates this problem.
According to the present invention, a method of assembling a piezoelectric actuator arrangement comprises the steps of:
providing a piezoelectric element having first and second ends;
arranging the piezoelectric element within a sleeve member such that the inner surface of the sleeve member and the outer surface of the piezoelectric element together define, at least in part, a chamber for receiving a first filler material;
injecting the first filler material into the chamber; and
applying a heating effect to the sleeve member to deform the sleeve member; and
sealing the sleeve member against the first filler material.
The present invention provides an advantage over conventional plastic over-moulding and metal-encapsulation techniques in that manufacturing costs are reduced. Additionally, an encapsulated actuator arrangement of reduced size can be formed.
Preferably, the method comprises the step of degassing the first filler material following injection thereof into the chamber so as to substantially remove any unwanted air or gas from the first filler material.
The method may comprise the further step of injecting a second filler material into the sleeve member prior to injection of the first filler material so as to fill any recesses or pockets within the sleeve member with the second filler material, which may otherwise trap air during injection of the first filler material, wherein the second filler material has a relatively high viscosity compared to the viscosity of the first filler material. For example, the first filler material may have a viscosity in the range between 400 and 800 mPas and the second filler material may have a viscosity in the range between 5,000 and 10,000 mPas.
Typically, at least one of the first and second filler materials may take the form of a curable resin. Typically, the sleeve member is formed from a fluoropolymer or plastic material.
The method preferably includes the further steps of arranging a shaft in the sleeve member such that angular movement of the shaft causes angular movement of the sleeve member and the piezoelectric element, providing means for applying a heating effect to the sleeve member and angularly moving the shaft such that the heating effect is applied substantially uniformly to the circumferential surface of the sleeve member.
The method may include the further step of moving the shaft in an axial direction as the shaft is moved angularly, so as to ensure a substantially uniform heating effect is applied to the circumferential surface of the sleeve member along its entire axial length.
If the shaft is hollow, the first filler material may be injected into the chamber through the hollow shaft. Alternatively, the sleeve member may be provided with an aperture through which the first filler material is injected into the chamber.
The method preferably comprises the further step of securing electrical connection means or an electrical connection arrangement, for applying a voltage across the piezoelectric element in use, to the first end of the piezoelectric element.
The method may comprise the further step of securing an end member to the second end of the piezoelectric element and applying protective means or a protective arrangement to at least one of the electrical connection means and the end member so as to avoid contamination of the end member and/or the electrical connection means by the first filler material.
Preferably, the protective means may take the form of protective members or plugs formed from a flexible material so as to facilitate ease of removal of the protective means from the end member and/or the electrical connection means.
The piezoelectric actuator arrangement may include a plurality of piezoelectric elements arranged in a stack, or may include a single piezoelectric element.
According to a second aspect of the present invention, a piezoelectric actuator arrangement for use in a fuel injector comprising an accumulator volume for receiving fuel, in use, comprises a piezoelectric element which is arranged within a sleeve member, an inner surface of the sleeve member and an outer surface of the piezoelectric element together defining, at least in part, a chamber containing a filler material such that, when the actuator arrangement is in use, a force due to fuel pressure within the accumulator volume is applied to the piezoelectric element through the sleeve member and the filler material.
Preferably, the sleeve is formed from a heat-shrinkable material which deforms to form a seal against the first filler material upon application of heat thereto during assembly of the arrangement.